Traditionally, most commercially available inkjet printers have a print engine which forms part of the overall structure and design of the printer. The body of the printer unit is usually constructed to accommodate the printhead and associated media delivery mechanisms, and these features are integral with the printer unit.
This is especially the case with inkjet printers that employ a printhead that traverses back and forth across the media as the media progresses through the printer unit in small iterations. Typically, the reciprocating printhead is mounted to the body of the printer unit such that it can traverse the width of the printer unit between a media input roller and a media output roller, with the media input and output rollers forming part of the structure of the printer unit. It may be possible to remove the printhead for replacement, however the other parts of the print engine, such as the media transport rollers, control circuitry and maintenance stations, are usually fixed within the printer. Replacement of these parts is not possible without replacement of the entire printer.
As well as being rather fixed in their design construction, printers employing reciprocating type printheads are relatively slow, particularly when performing print jobs of full colour and/or photo quality. This is due to the fact that the printhead must continually scan the stationary media to deposit the ink on the surface of the media and it may take a number of swathes of the printhead to deposit one line of the image.
Recently, ‘pagewidth’ printheads have been developed that extend the entire width of the print media. The printhead remains stationary as the media is transported past its array of nozzles. This increases print speeds as the printhead no longer needs to perform a number of swathes to deposit a line of an image. Instead, the printhead deposits the ink on the media as it moves past at high speeds. With these printheads, full colour 1600 dpi printing at speeds of around 60 pages per minute are possible. Such speeds were unattainable with conventional inkjet printers.
The nozzles and ejection actuators in these printheads are MEMS (micro-electromechanical systems) structures. Gas bubbles in these micron-scale ink chambers can prevent drop ejection. The compressible gas absorbs the pressure pulse from the actuator so ink is not forced through the nozzle. Bubbles can form in the chambers from ‘outgassing’ of the ink—dissolved gasses come out of solution and back into a gas phase. To guard against this, many ink cartridges seal the ink from air with an air tight ink bag that slowly collapses as the ink is drawn to the printhead. Unfortunately, the collapsed bag retains a significant amount of ink between its folds when the cartridge is deemed empty. Also, the flexible bags tend to have a resist certain amount of resistance to collapsing once it has shrunk below a certain size. The ejection actuators must draw the ink against this increased negative pressure which can alter the drop ejection characteristics of the nozzles.